1. Field of the Invention
This invention relates to a process for the catalytic dimerization of propylene to provide a dimerized product in an effluent which is predominantly hexenes. The major portion (by weight) of the hexenes are not only isohexenes, but more specifically, tert-isohexenes. By "isohexenes" we refer to all branched chain hexenes and not specifically to only those having a (CH3)2--CH-- group at the end of a hydrocarbon chain. By "tert-isohexenes" we refer to those isohexenes having an etherifiable C atom, namely, a C atom with a double bond, the C atom connected to two other C atoms, for example ##STR1##
The process employs the acidic form of certain natural or synthetic porous crystalline materials or zeolites as the catalyst, but only those more constrained small pore (sometimes generically referred to as "intermediate pore") zeolites having 10-membered rings, namely ZSM-22, ZSM-23, ZSM-35, and ZSM-48, which we found to have unique characteristics under the process conditions described herebelow. Such catalysts are therefore referred to herein as "chosen catalysts".
Though other catalysts may be synthesized to duplicate the unique shape selectivity of the foregoing "chosen catalysts", these chosen catalysts are the only ones we know will provide the peculiar catalytic activity which is the cornerstone of my process. Such activity is attributable in large part to the constraint index (CI) and sorption characteristics of the chosen catalysts, as will be described hereinafter.
The dimer from the effluent, referred to herein as "dimerized product" because it contains a major portion by weight of hexene isomers (dimer), is recovered with unexpected ease. The ease with which the dimer is obtained is fortuitous because the saving in processing costs provides one with the option either to blend it (the dimer) directly into "base" gasoline, or, to etherify all, or a portion of the dimer with lower C.sub.1 -C.sub.5 aliphatic alcohols, including secondary alcohols. The latter option is especially advantageous because the etherification reaction proceeds apace and with gratifying selectivity. The surprising economic effectiveness of the etherification process is directly attributable to the peculiar property of any chosen catalyst to isomerize non-tertiary hexenes to tert-hexenes and tert-isohexenes without producing an equilibrium mixture of common carbon number components, as will be explained herebelow.
The ease with which the etherification reaction proceeds, in turn, allows the etherate to be blended into gasoline. When "base" (C.sub.5.sup.= +) gasoline (RON 93.7; MON 79.1, for example) is boosted with the etherate of isohexenes, and of tert-isohexenes in particular, gasoline so blended has a highly useful octane number, and a relative low RVP (Reid Vapor Pressure) with respect to base gasoline.
It will be readily recognized that ethers of tertiary hexenes have a wide variety of uses, but one can only aspire to use them as a constituent of gasoline if the difficulty of making them economically can be overcome.
2. The Relevant Prior Art
The past decade has seen a great emphasis on upgrading light monoolefins by converting them to more valuable, higher molecular weight products. In particular, the olefin interconversion process converts C.sub.2 + monoalkenes into an equilibrium olefin mixture under conditions which maximize the formation of C.sub.4 and C.sub.5 iso-olefins (isobutene, 2-methyl-1-butene, and 2-methyl-2-butene). These tert-olefins react readily with methanol to form methyl ethers, namely methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) which are components for high octane gasoline.
The olefin interconversion process must cope with undesirable side reactions which yield aromatics and paraffins, the presence of which is acutely noticed at the relatively high temperatures (&gt;700.degree. F.) at which i-C.sub.4.sup.= and i-C.sub.5.sup.= formation is thermodynamically favored. Moreover, since the MOI process produces a near-equilibrium mixture of C.sub.2 -C.sub.13 olefins, it is necessary, in the prior art processes, to recycle the C.sub.6.sup.+ product to increase the selectivity to the desired C.sub.4 and C.sub.5 iso-olefins. But recycling the C.sub.6.sup.+ product which results in cracking and isomerization reactions can vitiate the economics of the recycle. Furthermore, the presence of H-transfer products and cycloolefins in the recycle can reduce overall yields under recycle conditions to the point where the economics of the process relegate it to be too demanding to be commercial.
In our invention, in addition to producing a predominantly hexene-containing dimerized product, more than 50% by weight of which hexenes are tert-isohexenes, our process has another unique functional characteristic. Process economics of etherification of tert-isohexenes dictate that unreacted C.sub.6.sup.= be recycled to the dimerization reactor (because they were not etherified). The chosen catalysts used in our process are uniquely able to isomerize the recycle to generate a mixed stream of tert-isohexenes and hexenes essentially in equilibrium with each other, the molar amount of the C.sub.6 s in the mixed stream being the same as that of the C.sub.6 s in the recycle stream. This isomerization of C.sub.6.sup.= olefins on a mol for mol basis has not been documented or otherwise substantiated, to our knowledge, for any zeolite catalyst. As a result of this unique activity of the chosen catalysts, there is essentially no loss of valuable C.sub.6.sup.= due to the formation of undesirable byproducts.
Though unreacted oligomers from prior art etherification reactors are recycled to the oligomerization reactor, the recycle stream in such prior art processes does not produce isomerized olefins in equilibrium with the unreacted recycled monoolefins on an essentially mol for mol basis. The foregoing facts about recycling unetherified olefins to the oligomerization reactor is implicitly emphasized in U.S. Pat. No. 4,886,925 to Harandi. The conversion of a feedstock rich in C.sub.2 + n-alkenes using a medium pore zeolite results in a first stream of C.sub.4 -C.sub.6 alkenes rich in isoalkenes, a second stream of C.sub.7 + olefinic gasoline boiling range hydrocarbons, and a third stream of unconverted hydrocarbons. It is commercially disadvantageous to deal with three such streams if the goal is to produce tert-alkyl ethers economically.
Clearly, it would be far more advantageous to oligomerize an olefin stream to produce only tert-isoalkenes which could then be converted to the desired ethers with great economy. But nothing in the prior art suggests how one might tailor a zeolite-catalyzed oligomerization process to produce a major proportion by weight of any tert-isoalkenes. In particular, there is no suggestion that one might efficiently oligomerize a substantially pure propylene stream to produce a major proportion of tert-isohexenes in the effluent, irrespective of the particular characteristics of the catalyst which may be used to do so.
A particular process described in U.S. Pat. No. 4,899,014 to Avidan, Johnson and Soto, discloses a process for conversion of a propylene-rich feedstock which contains at least 2 mol % ethylene into isobutane and C.sub.5 + gasoline. Another oligomerization process described in U.S. Pat. No. 4,873,385 to Avidan and Johnson describes the conversion of a propylene-rich feedstock to distillate using a wide variety of ZSM catalysts. Any ZSM-5 type catalyst having a constraint index (CI) in the range from 1 to 12, including ZSM-22, ZSM-23, ZSM-35, and ZSM-48, is said to be effective. The temperature and pressure at which this conversion occurs is stated to be in the range from about 315.degree. C. to 510.degree. C., and from 400 to 2500 kPa, respectively. Such conditions generally encompass the operating conditions for the olefin interconversion process, as well as the process of this invention. Yet the '014 process produces a gasoline range product containing at least 6% isobutane; and, the '385 process produces about 20% by weight of distillate per pass. There is essentially no distillate produced in the dimerization of a propylene-rich feedstock in our process because oligomerization to C.sub.9 + is less than 10% by weight of the effluent, and there is less than 6% isobutane.
There would seem to be good and sufficient reason to believe that, knowing the scope of the foregoing '925, '014 and '385 disclosures, one could use substantially the same catalyst, under substantially the same process conditions, to provide substantially the same result. If one did so, the result would be the formation of isobutane and C.sub.5 + hydrocarbons (in the '014 process), or, three streams which have to be dealt with (in the '925 process), or, gasoline and distillate. All such results are far removed from the goal of making a dimerized product, the major portion by weight of which is the dimer (hexene isomers); and, the more important goal--a dimer in which a major portion by weight of the hexenes is present as tert-isohexenes (in the effluent of an oligomerization reactor).
Despite the foregoing reasonable expectation, the best mode of the-process was found to be with a substantially C3 feed-stream, containing a major portion by weight of C3= and substantially free of ethylene, which was found to make the essential difference in the production of the dimerized product. By "substantially free of ethylene" we refer to a stream which has less than 5 mol % ethylene in it, preferably less than 2 mol %.
The dimerized product was produced in our process over only a few of the many ZSM-5 type catalysts having 10-membered rings suggested in the art as being effective oligomerization catalysts. ZSM-5 is disclosed in U.S. Pat. No. 3,702,886 and U.S. Pat. No. Re. 29,948.
Particularly because the activity of such catalysts is so closely tied to their physical structure, it is surprising that only some of those ZSM catalysts which have a constraint index (CI) in the range from 3 to about 10, and specifically only a few having a CI in the range from 3.5 to 9.1, have been found useful in the process claimed herein. ZSM-5 itself, with a CI of 6, is not. ZSM-22 with a CI of 7.3, is.
For the same reason, namely, that the catalytic activity of such catalysts is so closely tied to their physical structure and shape selectivity, it is surprising that only some (the chosen) ZSM catalysts are able to provide the unique physico-chemical characteristics found to be effective in my dimerization process, though others have about the same pore size (largest pore size in the range from about 4.2 .ANG..times.5.5 .ANG. to about 5.3 .ANG..times.5.6 .ANG.) and a sorption rate, measured at 100.degree. C., for n-hexanes in the relatively narrowly defined range from about 25-50 .mu.L/ (gm)(sec.sup.0.5) (.mu.L=microliters). ZSM-5 itself, which has a largest pore size of 5.3 .ANG..times.5.6 .ANG. and a sorption rate of 50 .mu.L/(gm)(sec.sup.0.5) is not. ZSM-48 which has the same largest pore size, namely 5.3 .ANG..times.5.6 .ANG., and a sorption rate of 32 .mu.L/(gm)(sec.sup.0.5), is.
In view of the fact that the dimerized product sought is tert-isohexenes which are large molecules about the same size as 3-methylpentane, one would expect that a zeolite with a large pore size, like ZSM-5, which exhibits a relatively high sorption rate of 3-methylpentane, would be far more effective dimerization catalysts, with better selectivity, than one with a relatively smaller pore size, like ZSM-35, ZSM-22 or ZSM-23. Yet ZSM-5 is not even closely competitive for the purpose at hand.
The foregoing peculiar relationship of sorption rate to the catalytic activity of the chosen catalysts, extends to the equilibrium sorption capacity ("ESC" for brevity, cc/g) they exhibit, as will be described hereafter.
Prior art oligomerization catalysts were never primarily concerned with producing a dimerized product with a substantially C3= rich feed. The problem they solved was not how to make dimer efficiently, but how to oligomerize C3=s to any oligomerization product efficiently. Since there was no reason for picking and choosing amongst the many effective oligomer-ization catalysts, the prior art provided no clue as to which criteria determined the unique effectiveness of any specific ZSM or related catalyst for the purpose at hand, namely dimerization. Since the narrow problem of making a major proportion by weight of C.sub.6.sup.= dimer (for over-riding economic reasons), was never isolated from the overall problem of making oligomerization product, prior workers found no reason to consider the cause of the narrow problem. It is therefore not surprising that they never suggested a solution to a problem they did not have.
Not at all coincidentally, despite the well-known fact that the economics of operating an oligomerization process is very much an essential facet, if not the most important facet, of its success, it is worth noting that the economic bottlenecks of the prior art processes are not highlighted. The economics of our process relies largely upon operating with a propylene-rich feed, essentially free of higher and lower olefins, containing at least 60% propylene and more preferably at least 80%, without benefit of a C3=/C3 superfractionator. Because our dimerization reactor ignores the presence of propane, it discharges the function of the missing superfractionator even if a single reactor is used, and a portion of the dimerized product stream containing unreacted propylene and incoming propane, is recycled. More preferably, since the per pass conversion of C3= and selectivity to hexenes are each more than 30% and preferably greater than 50%, the use of plural reactors in series, effectively converts essentially all propylene and obviates recycling the remaining propylene and propane. Thus, with plural reactors in series, the only recycle stream to the dimerization reactor can be unetherified hexenes.